Superconductive apparatus

ABSTRACT

Apparatus is provided whereby the conductive state of means including an element capable of exhibiting superconductivity is controlled by the maintenance of said means in intimate thermal communication with a paramagnetic salt. The temperature of such means thereby being discretely and selectively changed by the control of the magnetic state of said paramagnetic salt.

United States Patent Wright, Jr.

[ 5] Feb. 29, 1972 [54] SUPERCONDUCTIVE APPARATUS [72] Inventor: RobertC. Wright, Jr., 18 Lafayette Ave.,

Hingham, Mass. 02043 [73] Assignee: Wright, Robert C., Kendall Park, NJ.

[22] Filed: Aug. 6, 1968 [2!] Appl. No.: 750,568

[52] US. Cl. ..307/240, 321/8 CD, 321/49, 307/245, 307/277, 307/306,307/296 [51] Int. Cl. ..H03k 17/00 [58] Field of Search ..32ll49, 8 CD;307/245, 240; 330/4 [56] References Cited UNITED STATES PATENTS3,080,527 3/1963 Chester....,. "330k! To Cryogenic Lood 3,398,299 8/1968Walkeret al. ..307/245 3,505,538 4/1970 Artleg Primary Examiner-Roy LakeAssistant Examiner-Darwin R. Hostetter Attorney-Mam & Jangarathis [57]ABSTRACT Apparatus is provided whereby the conductive state of meansincluding an element capable of exhibiting superconductivity iscontrolled by the maintenance of said means in intimate thermalcommunication with a paramagnetic salt. The temperature of such meansthereby being discretely and selectively changed by the control of themagnetic state of said paramagnetic salt.

4 Claims, 7 Drawing Figures PAIENTEDFEB 29 I972 SHEET 1 OF 3 INVFNTORRober? C. Wright M148 4 a: I F

ATTORNEYS PATENTEUFEB29 I972 3. $46,363

SHEET 2 OF 3 INVENTOR. Robert C. Wright ATTORNEYS PATENTEDFEBZSIQYZ3,646,363

SHEET 3 OF 3 To Cryogenic Loud 4A Normal Range a "2 E H g H MawATTORNEYS SUPERCONDUCTIV E APPARATUS This invention relates tosuperconductive devices and more particularly to a novel superconductiveswitch which may be instantaneously changed from a first conductivestate to a second conductive state in a substantially adiabatic manner.Additionally, this invention relates to particular apparatus utilizingone or more of said superconductive switches to provide power or logicfor other superconductive devices present in a cryogenic environment.

It is well known that the electrical resistance of most materialsgenerally decreases with a decrease in the temperature of the material.However, it is equally well known that a certain class of materials,generally referred to as superconductors, while following theabove-mentioned rule above a critical temperature, suddenly exhibitessentially zero electrical resistance when the temperature of suchmaterials is lowered below said critical temperature. This class ofmaterials is made up of many metallic elements as well as metalcompounds and alloys, each of which has a clearly defined criticaltemperature at which the resistance suddenly becomes essentially zero.The majority of metallic element superconductors usually have separatecritical temperatures which reside only a few degrees above absolutezero K.); however, present research has produced superconductivecompounds and alloys having clearly defined critical temperatures ashigh as 18 K. Additionally, such superconductive materials, when belowtheir critical temperature, exhibit the Meissner effect in that theyappear to be diamagnetic as to magnetic field strengths below a givenvalue, which value tends to increase as the temperature below thecritical temperature is decreased. Once, however, the magnetic fieldstrength is increased beyond this critical value, the zero resistancestate is destroyed. Thus, the value of current that a superconductivedevice may carry while remaining in the zero resistance state istheoretically limited only to the maximum at which the current flowingthrough the superconductive device creates a magnetic field at thesurface thereof which field approaches the critical mag netic fieldstrength.

The attributes of such superconductive devices have proved to beattractive in regard to uses within the data storage field, in magnetsystems, and other fields where the maintenance of a constantcirculating current, often referred to as a persistent current, is adesirable feature. Such uses, however, have been greatly curtailed dueto present problems associated with the maintenance of a predeterminedcryogenic environment which environment is normally provided bysurrounding such devices in liquid helium contained in Dewar typeapparatus. Thus, an isolated cold zone is utilized to maintain thesuperconductive apparatus at the selected temperature.

Apparatus which is presently relied on to provide the cold zone forsuperconductive devices has been found to be highly expensive andinefficient in operation because such apparatus is imperfectly isolatedfrom ambient conditions and also because of the heat produced within thecryogenic environment. A principal deficiency in the isolation of thecold zone has been traced to the use of relatively large copperconductors to connect the superconductive devices present in the coldzone with apparatus external thereto. Such copper conductors have beenfound to continuously transmit thermal energy to the cold zone from theexternal environment which energy in the form of heat must becontinually dissipated by the cryogenic environment. Furthermore, thesame copper conductors, due to their finite resistance, cause jouleheating to take place within the area of the cold zone when currentflows therein. Although not serious in low current applications, suchjoule heating has proved to be a severe system deficiency where largecurrents are applied as in superconductive magnets which utilizehundreds of amperes. Thus, the thermal energy introduced into the coldzone causes boiloff gas cooling to take place at the surfaces of therelatively large copper conductors present in the cold zone therebyrendering the operation of the apparatus for maintaining a cryogenicenvironment highly expensive and inefficient.

Therefore, it is a principal object of the present invention to providea substantially adiabatic, superconductive switch for use inside thecold zone which may be rapidly cycled between two states, one of whichis superconductive thereby manifesting a zero voltage drop and no jouleeffect heating and a second state exhibiting normal, finite resistance;whereby the selection of one of such states is determined by thepresence or absence of a relatively small magnetic field, enabling theuse of relatively small copper conductors with their inherently smallertotal of heat conveyance into the cold zone, which presence or absenceof said magnetic field causes essentially adiabatic temperature changesto take place in said switch thereby reversibly switching the same withno net heat input into the system.

A further object of this invention is to provide superconductive logicapparatus exhibiting substantially adiabatic switching properties whichinherently has no joule effect heating regardless of the current levelspresent therein.

A further object of this invention is to provide superconductive powersupply apparatus manifesting substantially adiabatic switchingcharacteristics which is highly efficient and causes only relativelyinsignificant amounts of heat to be produced within the cryogenicenvironment.

A further object is to provide high frequency chopper apparatus for useinside the cold zone wherein closely regulated temperature control isrendered unnecessary.

Other objects and advantages of the invention will become clear from thefollowing detailed description of several embodiments thereof, and thenovel features will be particularly pointed out in connection with theappended claims.

The present invention makes use of the unique physical properties ofparamagnetic salts to instantaneously change the temperature of anelement in thermal communication therewith without the insertion orremoval of any net thermal energy into or from a closed system. Althoughmost sub stances are magnetically neutral, paramagnetic salts havemagnetic domains present therein which normally have a randomorientation, but which are capable of being aligned. Thus, even though aparamagnetic salt crystal is usually magnetically neutral due to the netfield cancellation between the randomly orientated domains, in thepresence of an external field, the magnetic domains present therein willalign themselves such that the salt crystal will manifest a magneticpolarity. Since the electrons present in such an aligned crystal ofparamagnetic salt are at a lower energy level than those of a randomlyorientated crystal, heat is given off to the environment when thedomains of such a crystal are aligned, and heat is absorbed when thefield is removed and the domains are again allowed to relax. Such heattransformations can be made adiabatic if the crystal, in either itsnonaligned state or its aligned state, is at the same temperature as itsenvironment. Furthermore, discrete temperature variations can beproduced according to the Gibbs Rule with essentially no net input oroutput of thermal energy from the system. Thus, this invention makes useof the instantaneous, adiabatic change in temperature of such crystalsto switch or to augment the switching of a superconductive devicebetween discrete temperatures to thereby cause the superconductivedevice to become selectively superconductive or nonsuperconductive.

Therefore, in accordance with a first aspect of this invention, asuperconductive switch is provided wherein the state of asuperconductive device is instantaneously controlled in an adiabaticmanner by the magnetic condition of a paramagnetic salt in intimatethermal communication therewith.

According to a second aspect of this invention, a superconductivemultivibrator is provided wherein the binary state of the multivibratoris controlled in a substantially adiabatic manner by the magneticcondition of a plurality of paramagnetic salts in thermal communicationwith the superconductive elements thereof.

Additionally, in accordance with a third aspect of the presentinvention, a power supply for superconductive devices is providedwherein the operation of the power supply is adiabatically controlled bythe magnetic state of a plurality of superconductive devices.

Furthermore, in accordance with a fourth aspect of the present inventiona high frequency chopper is provided wherein the operation of asuperconductive element is relied upon to enable wide temperaturevariations in the cryogenic environment surrounding said superconductiveelement.

Further objects and aspects of the present invention will be apparentfrom the operation of the embodiments of the instant invention which aredisclosed herein. The operation of the disclosed embodiments of thepresent invention will be clearly understood from the followingdescription and the accompanying drawings in which:

FlGS. 1A and 1B show preferred forms. of superconductive switchesaccording to the present invention;

FIG. 2 embodies a preferred form of a multivibrator device made inaccordance with the present invention;

FlGS. 3A and 38 indicate preferred embodiments of a superconductivepower supply as contemplated by the present invention; and

FIG. 4A shows a preferred form of chopper apparatus made in accordancewith the present invention, while FIG. 48 depicts the operatingcharacteristics thereof.

The superconductive switch 1 of the FIG. 1A embodiment comprises asuperconductive ribbon 2, paramagnetic salt controlling means 4, and acoil 6. The superconductive ribbon 2 may be formed of any of thewell-known class of superconductive materials and although it is shownas a ribbon, it may take any readily available shape. The ribbon 2 whichforms the conductive element of the switch 1 has input and output leads8 and 10 respectively connected thereto. The leads 8 and 10 connected tothe superconductive ribbon 2 may be made of ordinary conductor materialbut are preferably formed of a hard superconductive material which has ahigher critical temperature than the superconductive ribbon 2. Thus, anytwo superconductive materials having several degrees difference betweentheir respective critical temperatures may be used, for instance, if thesuperconductive ribbon material 2 is made of Nb Al which has a criticaltemperature of approximately K., the leads 8 and 10 may be formed of NbSn which has a critical temperature of approximately 18 K. In such acase the material of the leads 8 and 10 would be considered hard and thematerial of ribbon 2 would be considered soft.

The superconductive material of the leads 8 and 10 is selected to beabove the critical temperature of the ribbon element 2 so that, as willbe shown hereinafter, regardless of the state of the ribbon element 2,the leads are always in a superconducting state. The superconductingleads are here preferred to insure that no joule heating takes placewithin the system.

The paramagnetic salt controlling means 4 may either be a singleparamagnetic crystal grown around the superconducting ribbon 2 orpowdered crystals packed about the said ribbon 2. Any paramagnetic saltwhich is readily available may be utilized, such as gadolinium sulfate,cerium fluoride, dysprosium ethyl sulfate, cerium ethyl sulfate,chromium potassium alum, iron ammonium alum, alum mixture, cesiumtitanium alum, manganese ammonium sulfate, gadolinium nitrobenzenesulfonate, or copper potassium sulfate. The paramagnetic saltcontrolling means 4 is wound by a plurality of turns of the coil 6 whichmay be made of ordinary conductor material or a superconductive materialhaving a critical temperature similar to that of leads 8 and 10.Although a coil 6 has been shown, it should be noted that a selectablyenergized magnet could be substituted therefor to apply the requisitefield to the paramagnetic salt controlling means 4. The entire switchapparatus 1 as well as the remainder of the superconductive devicesordinarily connected thereto are maintained within a cryogenicenvironment which may be supplied by any of the well-known systemspresently in use. Since such cryogenic systems are well known and formno part of the present invention per se, the system utilized is merelyindicated by the dashed block 12 in FIG. 1A and is not hereinaftershown.

In the description of the operation of the HO. 1A switch embodiment ofthe present invention which follows, it will be understood that acurrent source, which may be any standard source capable of providingthe necessary current value, will be connected to the input lead 8 at apoint preferably outside of the cryogenic environment 12; and asuperconductive utilization device will be connected to the output lead10 of the switch 1. Additionally, it is to be understood that a currentsource is to be connected to the leads of the coil 6 so that the coil 6can selectively provide a sufficient magnetomotive force (H) to theparamagnetic salt controlling means 4 to change the temperature of thesuperconductive ribbon 2 by approximately 1 or 2 Kelvin. Such atemperature change is desirable to insure that the superconductiveribbon element 2 is switched clearly across the critical temperaturerange thereof. The magnetomotive force H, which must be applied to theparamagnetic salt to obtain the necessary change in temperature may becalculated by using the relationship that:

H, the magnetomotive force in kilooersteds T the temperature with thefield on in degrees Kelvin T the temperature with the field off indegrees Kelvin A is a constant and C is the heat capacity of the salt.The ratio of All? for gadolinium sulfate and chromium potassium alum,two of the more well-known paramagnetic salts, was found to beapproximately 0.5 and 1.3, respectively.

With the switch 1 of FIG. 1A connected as stated above, two separate,initial modes of operation, depending on the selected temperature of theenvironment present within the cryogenic system 12 are possible. If itis assumed for the purposes of explanation, that the criticaltemperature of the superconductive ribbon 2 is 15 K. and that thecryogenic environment is maintained at 16 K., a first mode of initialoperation of the superconductive switch 1 is dictated. in this firstmode of operation, the coil 6 is in the normally energized condition andthe system is allowed to initially equalize in temperature such that thecryogenic environment, the superconductive ribbon 2, the leads 8 and 10and the coil 6 are all maintained at the temperature of the cryogenicenvironment which, as stated above, is 16 K. Since the superconductiveribbon element 2 is above its critical temperature, it is in arelatively high resistance state and thus the superconductive switch 1is in its open condition. Thereafter, the normal resistive state of theribbon element 2 will be maintained by the current generated by coil 6even if a slight shift in the temperature of the cryogenic environmentshould occur. When it is desired to place the superconductive switch 1in the closed condition, the current source which was driving the coil 6is deenergized thereby causing the magnetic field which had been appliedto the paramagnetic salt control means 4 to collapse. When the magneticfield is removed from paramagnetic salt control means 4, the previouslyaligned domains present therein will tend to return to their normalstate of random orientation which requires additional energy. Thus, theparamagnetic salt, exhibiting the well-known magnetocaloric effect,withdraws this energy from its environment and instantaneously reducesthe temperature of the superconductive ribbon element 2 in intimatethermal communication therewith in a substantially adiabatic manner.Where magnetomotive force applied to the paramagnetic salt control meanswas calculated to be sufiicient to change the temperature thereof by 2K., the temperature of the ribbon element 2 is lowered by this amountand is thus at approximately 14 K. which is well below its criticaltemperature. The superconductive ribbon element 2 is thus in itssuperconductive state and the superconductive switch 1 is in its oncondition thereby passing the current present at its input lead 8 to theutilization device connected to its output lead 10 via a zero resistancepath. Thus, the superconductive ribbon 2 of the switch I has beendiscretely and selectively switched from its nonsuperconductive state orof? condition to its superconductive state or on condition via theapplication thereto of a predetermined, instantaneous, and precisetemperature change.

The superconductive switch 1 is returned to its off condition by againenergizing the current source connected to the coil 6 which is woundabout the paramagnetic salt control means 4. When the current source isagain energized, a current begins to flow in coil 6 which is of asufficient magnitude so that the coil 6 applies the previously specifiedmagnetomotive force to the paramagnetic salt control means 4. Thismagnetomotive force will tend to cause the randomly oriented domainspresent in the paramagnetic salt control means 4 to align with theapplied field so that said paramagnetic salt control means 4 no longermanifests an overall neutral polarity. The degree to which the totalnumber of domains align with the field is determined by the magnitude ofthe magnetomotive force applied by the coil 6. The electrons present inthe domains of the paramagnetic salt, which align, will drop to a lowerenergy state thereby releasing thermal energy to the environment. Thethermal energy which is thereby released is a function of the degree towhich the total number of domains align and is thus a function ofmagnetomotive force applied by the coil. This magnetomotive force, asspecified above, will liberate sufficient energy to the environment sothat the superconductive ribbon 2 increases in temperature by 2 K.thereby being raised to a temperature of 16 K. which is above itscritical temperature. The superconductive ribbon 2 is thus renderedresistive and hence the superconductive switch 1 is placed in its offcondition and will remain in said off condition, due to the presence ofthe magnetic field, even if small temperature changes should occur.Thus, it will be seen that the superconductive switch I has beeninstantaneously returned to the off condition in a substantiallyadiabatic manner.

A second mode of initiating operation is possible for the FIG. 1Aembodiment of the present invention. In this mode of operation, adifferent set of initial conditions are present, however, once they areestablished, the superconductive switch I is operated in the manner aspreviously specified. In this case, the superconductive switch 1 isconnected to the two current sources and the utilization devicementioned above but, assuming the same critical temperatures mentionedabove, the cryogenic environment is initially established at 14 K. andthe coil 6 is not energized. Therefore, the isolated cryogenic system isallowed to equalize at a temperature of 14 K. and the superconductiveribbon 2 is initially in the superconductive state and thus thesuperconductive switch 1 is initially in the on condition. Thereafter,the superconductive switch 1 may be placed in the off condition by theenergization of the current source connected to the coil 6. Theenergization of the coil 6 causes a magnetomotive force to be applied tothe paramagnetic salt control means 4 in the manner previously specifiedthereby causing a portion of the domains therein to align with theapplied field and adiabatically release thermal energy to the system.Thus, the superconductive switch 1 is instantaneously switched in anadiabatic manner to its off condition in the same manner as mentionedabove. When it is again desired to change the state of superconductiveswitch I to the on condition, the current source connected to the coil 6is deenergized thereby causing the domains of the paramagnetic salt 4 toregain their random orientation and remove thermal energy from thesystem in the same manner mentioned above. Thus, although the initialconditions present in the above described second mode of operationdiffer from those of the first mode of operation stated above, theoperation thereafter is the same. Therefore, it will be seen that asuperconductive switch has been provided which can be adiabatically andinstantaneously controlled by the magnetic condition of a paramagneticsalt in thermal communication therewith.

It should be apparent that although the FIG. 1A embodiment has beendescribed in conjunction with a superconductive ribbon 2, any form orshape of a superconductive element may be used therefor. Thus, a wire orany other usable shape may be utilized and since superconduction isgenerally accepted to be surface conduction, the superconductivematerial may be conserved by the utilization of hollow conductors. Inaddition, it should be apparent that although the superconductiveelement has been shown threaded directly through the paramagnetic saltcontrol means 4, if large currents are to be used therein, thesuperconductive element could be doubled back on itself to therebycancel the field induced in the salt by such currents. Furthermore, aplurality of superconductive elements may be utilized within a singleparamagnetic salt control means to create a multiple pole device.Additionally, it should be appreciated that although the superconductiveribbon element 2 of the switch 1 has been shown positioned within theparamagnetic salt control means to link the field applied by the coil 6,which field thereby aids the switching due to the lowering of thecritical temperature of the superconductor as mentioned above, thesuperconductive element may be located perpendicular to the field sincefield linkage is not mandatory to the successful temperature switchingutilized by this invention, provided that a truly closed thermal systemcan be approached. Finally, it should be pointed out that the terminstantaneous as used herein is intended to cover the very fastswitching time of the disclosed embodiments of the instant inventionwhich should enable operation in the nanosecond range; however, sincetime is not generally a thermodynamic parameter, this speed has not beencalculated.

The superconductive switch 14 shown in FIG. 1B is a noninductiveembodiment of the superconductive switch shown in FIG. 1A. Theparamagnetic salt control means 4, the coil 6, the input lead 8 and theoutput lead 10 utilized with the FIG. 1B embodiment may be identical tothose described with regard to the FIG. 1A embodiment and therefor havebeen given the same reference numerals. The superconductive switch ofFIG. 18 comprises the same elements as described with regard to the FIG.1A embodiment except that a twisted superconductive wire element 16 issubstituted for the superconductive ribbon 2 of FIG. 1A. The operationof the FIG. 1B embodiment is as described with regard to the FIG. 1Aembodiment and as such will not be repeated here. However, when the FIG.1B switch 14 is in its on condition, with current thereby passing fromthe input lead 8 to the output lead 10 through superconductive element16, any field generated by said current will be canceled. Suchcancellation takes place because equal currents pass through the twotwisted halves of the superconductive wire element 16 in oppositedirections such that the fields generated thereby will be of equalmagnitude but opposite in direction and therefore will cancel. Thisembodiment is preferable when a switch having more than onesuperconductive element therein is utilized so that there is noinductive coupling between superconductive switch elements in intimatethermal contact with the same paramagnetic control means 4.Additionally, this embodiment is useful when high currents are to bepassed through the switch which currents might tend to generate a fieldcapable of aligning the domains of the paramagnetic salt means 4.

The superconductive multivibrator shown in FIG. 2 is a binary logicdevice which utilizes two of the superconductive switching elements ofFIG. IA. Accordingly, similar switch elements have been given previouslyutilized notations; however, since the multivibrator is a symmetricaldevice, each numeral used therewith is followed by a letter designationL or R to indicate the portion of the symmetrical circuit to which itbelongs.

The multivibrator of FIG. 2A comprises two superconductive switches 1Land IR each of which includes, respectively, a superconductive ribbonelement 2L and 2R, a paramagnetic salt control means 4L and 4R, and acoil 6L and 6R, which should be made of superconductive material. Thesuperconductive ribbon elements of each switch 1L and IR are connectedto input leads 8L and SR and output leads 10L and 10R, which leads inthis case should be formed of hard superconductive material having ahigher critical temperature, as explained with regard to FIG. IA, thanthe critical temperature of the superconductive ribbon elements 2L and2R. The

output lead L of the switch 1L has an output terminal I,,, connectedthereto and is additionally connected by the lead 16 to one terminal ofthe switching coil 6R of switch 1R. In similar manner, the input lead 8Rof the switch 1R has an output terminal I connected thereto and isadditionally connected by the lead 18 to one terminal of the switchingcoil 6L of switch IL. The other terminal of the switching coil 6R of thesuperconductive switch IR is connected by lead to switch 8, which in afirst position connects to ground G, and in a second position connectsto the junction between the input lead 8L of superconductive switch 1Land the switch 5,. The switch 8,, when in its closed position, connectsthe aforementioned junction to the output of the current source I, whichmay be any well-known type of current supply. The output lead 10R of thesuperconductive switch IR is connected by a conductor 22 to a firstterminal of switch S The switch S connects in a first position to groundG and at a second position thereof connects to the junction between afirst terminal of switch 5., and a second terminal of the switching coil6L of the superconductive switch IL. The switch S, when in its closedposition connects to the output terminal of current source I which maybe similar in nature to current source 1,, or even a separate,selectable output thereof. The connectors l6, 18, 20 and 22 should bemade of a suitably hard superconductive material, similar to thatutilized for the leads 8L, 8R, 10L and 10R and having a similar criticaltemperature. The entire apparatus, except for the current sources aremaintained in a cryogenic environment, not shown, similar to thatutilized with regard to the FIG. 1 embodiment. Additionally, eachparamagnetic salt control means 4L or 4R may have a suitable insulatingcoating such as Teflon thereon, not shown, so that each control means 4Lor 4R, together with its respective ribbon elements 2L or 2R may bepartially insulated from the overall cryogenic environment therebyenabling the separate temperature of each to be maintained and adiabaticoperations approached. The switches S,-S.,, which are shown as manualswitches for simplicity, may be of any suitable type of electronicswitches which are well known in the art of they may be the switches ofthe FIG. 1 embodiment of the instant invention. It should be appreciatedhowever, that switches S, and S should have superconductive elementstherein and if the switches of the FIG. 1 embodiment were utilized, aseparate switch should be substituted for each position of the twoposition switches S, and 5,. Furthermore, it should be noted that if thesuperconductive switches as shown in FIG. 1 are utilized, multielementsuperconductive switches could be used where two or more switchesoperate in unison.

In operation, the multivibrator of FIG. 2 may be used to provide a logicoutput or to provide storage for a single bit of binary information. Themode of operation of the switches 1L and IR therein is as explained withregard to FIG. I. and thus the following detailed explanation of theoperation of the multivibrator will only state that a selected switchingcoil 6L or 6R is energized or deenergized and the selected switch 1L or1R is thereby placed in its off or on condition. For the purpose ofexplanation, it may be assumed that the cryogenic environment is set ata sufficiently low temperature so that one of the superconductiveswitches 1L or IR, having its switching coil 6L or 6R deenergized willbe superconductive or on while the other superconductive switch with itsswitching coil energized will be nonsuperconductive or off. The offcondition wiil thereafter be maintained by the flux generated by thecurrent flowing in the energized coil. With this cryogenic environmentestablished, all superconductive elements within the circuit with theexception of those inside the superconductive switches will besuperconductive because the temperature will be below their criticaltemperature which is above that of the superconductive ribbon elements2L and 2R. It is additionally assumed for the purpose of explanationthat the switches S,S,, are initially in positions such that S, isclosed, S, is open, S, is at G, and S, is at 0,. With the switches inthese positions, a current path is established from the current sourceI, through the closed switch S, to the input lead 8L of thesuperconductive switch IL. The superconductive ribbon element 2L of thesuperconductive switch IL is in the superconductive condition, since, aswill be shown hereinafter, no current passes through the switching coil6L thereof. Therefore, the current from the source I, which is presentat input lead 8L passes through the superconductive ribbon element 2L ofsuperconductive switch 1L to the output lead 10L thereof. The currentpresent at output lead 101. is connected via the connector 16 to theswitching coil 6R of the superconductive switch IR thereby maintainingthe superconductive ribbon element 2R thereof in the nonsuperconductivecondition and hence the superconductive switch IR is in the offcondition. The current from the current source 1,, present in theswitching coil 6R, thereafter passes via conductor 20 and switch 5, toground at G,. No current from current source I, is, under theseconditions, circulating in the multivibrator circuit of FIG. 2 becausethe source I, is disconnected therefrom and any circulating currentswould be dissipated in the superconductive switch IR which is presentlyin the off condition or by the connection of the I, current loop toground at G via switch 8,. As soon as the I, current has beenestablished in the current loop which includes elements 8,, 8L, 2L, 10L,l6, 6R, 20 and 8,, the switch S, may be opened and the switch S, may beswitched to its second position to thereby connect the conductor 20 tothe junction between switch S and input lead 8L. As should be apparent,the current flowing in this closed loop which includes elements 8L, 2L,10L, I6, 6R, 20 and S, will continuously circulate therein in thecounterclockwise direction because the loop consists entirely ofsuperconductive elements and therefore has zero resistance. Thus, acontinuously circulating, persistent current has been established in thel, current loop which current maintains the superconductive switch 1R inthe off condition and represents a first binary logic or informationstate of the multivibrator. This persistent current will continuouslyflow in the previously defined loop for months unless the state of themultivibrator is first changed. The persistent current may be sensed orutilized directly by the selective connection of a utilization device tothe output terminal I,,, thereby destroying the information state orsensed indirectly by placing a coil about one of the conductors in thecurrent loop.

When it is desired to change the logic state or insert new binaryinformation into the superconductive multivibrator of FIG. 2, the switchS, is switched to its grounded position at G, and switch S, isthereafter placed in the closed position. The grounding at G, of switchS, breaks the previously closed superconductive loop and grounds thesame, thereby dissipating the persistent current which had beencirculating therein. The closure of switch S, completes a current pathfrom the current source I: to the switching coil 6L of thesuperconductive switch IL thereby placing it in the off condition. Thecurrent thus applied to the switching coil 6L of the superconductiveswitch IL is thereafter applied by conductor 18 to the input lead 8R ofthe superconductive switch 1R. Since there is no longer current presentin the switching coil 6R of the superconductor switch 1R, said switch IRis in the on condition so that current present at input lead SR ispassed through the now superconducting ribbon element 2R to the outputlead 10R. The current present at the output lead 10R is applied viaconductor 22 to one terminal of switch 8,, which has remained connectedto ground G,. Once the 1 current has been clearly established in thepreviously defined path and I, current has settled, switch S is openedand switch S is switched from position G to the position whereby itconnects conductor 22 to the junction between switch S, and switchingcoil 6L of the superconductive switch 1L now in the off condition. Thus,a second closed superconducting current loop, which includes elements6L, 18, 8R, 2R, 10R, 22 and 8,, has been established having a clockwisepersistent current continuously circulating therein. This persistentcurrent which will continue for several months unless interruptedmaintains superconductive switch IL in the off condition. It too may bedestructively sensed or utilized by a selective connection of autilization device to terminal I or indirectly readout by the placementof a coil about one of the current carrying elements therein. When it isagain desired to change the logic or information state of thissuperconductive multivibrator, the previously outlined steps with regardto the I current loop may again be initiated. It should be noted that ifadditional current magnitudes are deemed desirable in conjunction withthe operation of this multivibrator, the incorporation of a drivewinding for use therewith is specifically contemplated. Thus, it can beseen that substantially adiabatic, instantaneously switching,superconductive logic or information storage apparatus, which inherentlyhas no joule effect heating, has been provided according to the instantinvention.

The embodiment of the present invention which is depicted in FIGS. 3Aand 3B is a full wave rectifying power supply for a superconductivesystem usable within a cryogenic environment. The overall operation ofthis full wave rectifying power supply is shown in diagrammatic form inFIG. 3A while FIG. 38 indicates a specific form of apparatus usabletherein.

The full wave rectifying power supply of FIG. 3A comprises inputterminals 24 and 26, a full wave rectifying bridge 28, and outputterminals 30 and 32. The rectifying bridge 28 includes four arms A-Dtherein and each arm includes a superconductive switching element orswitch 34-37, respectively, which may be of the form shown in FIG. 1.The details of the switching elements 34-37 have not been shown in FIG.3A in order to avoid initial confusion, however, such details are fullyspecified with regard to FIG. 3B. The full wave rectifying bridge 28 isconnected to the input terminals 24 and 26 via conductors 40 and 38which are respectively connected to the bridge input terminals 42 and44. The output terminals 30 and 32 are connected to the bridge outputterminals 46 and 48, respectively, by conductors 50 and 52. Theconductors 38, 40, 50 and 52 may be made of ordinary conductivematerial, however, it is preferred that they be formed of hardsuperconductive material having a higher critical temperature than thesoft superconductive switches or switch elements 34-37. The entireapparatus of FIG. 3A is contained within a cryogenic environment whichhas not been shown.

The explanation of the operation of FIG. 3A will assume that a standardsource of alternating current is connected to the input terminals 24 and26 and that a cryogenic utilization device such as a superconductivemagnet is connected to the output terminals 30 and 32 thereof. Thealternating current applied to the input terminals 24 and 26, asindicated by the waveform 54, is applied to the bridge input terminals42 and 44 by conductors 40 and 38, respectively. The superconductiveswitches or switch elements 34-37 of the bridge arms A-D are switched onor off in pairs, by means more fully described with regard to theembodiment of FIG. 38, such that elements 34 and 36 are in their onstate when switches 35 and 37 are in their of? state and the conversethereof. Thus, the bridge arms A and C are rendered superconductivewhile bridge arms B and D are nonconductive and bridge arms B and D arerendered superconductive when bridge arms A and C are nonconductive.When bridge arms A and C are superconductive, and arms B and D arenonconductive, a superconductive current path will be established fromthe bridge input terminal 42 to the bridge output terminal 46 via arm Awhile a similar path will be established from bridge input terminal 44to bridge output terminal 48 via arm C. During such a time interval,other possible conductive paths between the input terminals 42 and 44and output terminals 46 and 48 via arms B and D will be foreclosed dueto the nonsuperconductive condition of switch elements 35 and 37. Whenbridge arms B and D are conductive and arms A and C are nonconductive, asuperconductive path will be established from the bridge input terminal42 to the bridge output terminal 48 via arm D while a similar path willbe established from bridge input terminal 44 to bridge output terminal46 via arm B. During this time interval, other possible conductive pathsbetween the input terminals 42 and 44 and output terminals 46 and 48 viaarms A and C will be foreclosed clue to the nonsuperconductive conditionof switch elements 34 and 36. The aforementioned switch element pairsare gated, as explained in more detail hereinafter, such that bridgearms A and C are superconductive during the time intervals that positivepulses are applied by the current source and bridge arms 8 and D aresuperconductive during the time intervals that negative pulses areapplied by said current source. Thus, the circuit acts as a full waverectification bridge and the alternating positive and negative pulsesapplied thereto are rectified and applied by the bridge output terminals46 and 48 to the output terminals 30 and 32 as indicated by the waveform56.

FIG. 3B shows one structural embodiment of the apparatus which may beutilized in the construction of the bridge 28 of FIG. 3A. The samereference annotations used in FIG. 3A have been retained in FIG. 38 sothat corresponding portions of each circuit may be easily identified.The rectifying bridge of FIG. 38 comprises bridge input terminals 42 and44, superconductive switches 58 and 60 and bridge output terminals 46and 48. The superconductive switches 58 and 60 are the noninductive typeof superconductive switches described hereinabove with regard to FIG.18, however, each switch has superconductive twisted wire elementstherein which correspond to the elements of the opposite sides of thebridge that are switched together. Thus, superconductive switch 58contains the superconductive switch elements 34 and 36 of arms A and C,respectively, while superconductive switch 60 contains thesuperconductive switch elements 35 and 37 of arms B and D, respectively,and inductive coupling between opposite bridge arms is avoided. Each ofthe superconductive switches 58 and 60 includes paramagnetic saltcontrol means 62 and 64, respectively, and switching coil means 66 and68 all of which are fully described in conjunction with FIG. 1. Again,the paramagnetic salt control means 62 and 64 may each include aninsulating coating to maintain the temperature thereof, together withtheir respective switch elements as well as to aid in adiabaticoperation. Each of the superconductive elements 34-37 is connectedwithin its requisite bridge arm A-D, respectively, in the mannerpreviously specified with regard to FIG. 3A so that input terminals 42and 44 and output terminals 46 and 48 form a bridge configuration.

Since the mode of operation of the overall bridge 28 was stated withregard to FIG. 3A, and the mode of switching of an individual switch wasdescribed with regard to FIG. 1, only the mode of switching the bridgearms in sequence with the input signals will be described hereinbelow.As was previously described, the bridge circuit is to operate such thatarms A and C are superconductive during the time intervals whichcorrespond to the positive pulses of the waveform 54 while arms B and Dare to be superconductive during the time intervals which correspond tothe negative pulses of said waveform 54. Thus, if the time intervalscorresponding to the positive pulses are those which begin at oddintervals t,, t; and end at even intervals and those corresponding tothe negative pulses are those which begin at even intervals 1 t, and endat odd intervals such as 1 it is only necessary to gate thesuperconductive switches 58 and 60 such that switch 58 is in the oncondition during the positive pulse time intervals and off during thenegative time intervals while the converse of this operation is utilizedwith regard to superconductive switch 60. The required synchronoussignals are shown by waveforms 70 and 72 as being applied to switchingcoils 66 and 68 of the superconductive switches 58 and 60, respectively.The synchronous signals may be supplied by any standard pulse sourceswell known in the art and, as should be apparent from the operationdescribed with regard to FIG. 1, when a pulse is applied to one of theswitching coils 66 and 68 the superconductive switching elements 34-37within the respective superconductive switches 58 and 60 are renderednonsuperconducting until that pulse terminates. Thus, it is seen thatthe bridge apparatus depicted in FIG. 38 provides alternate switching ofthe opposite arms of the superconductive bridge of FIG. 3A so that fullwave rectification is provided as shown by the waveform 74.

Although the full wave rectifying bridge embodiment of the instantinvention has been described in terms of a cryogenic power supply, itshould be obvious that similar structure thereto can be used as acryogenic DC to AC converter where the superconductive switches areexternally driven. Additionally, where such power supplies are utilizedto supply high currents for use in powerful superconductive magnets,further joule heat loss reductions may be achieved by supplying highvoltage, low current power to the input of the superconductive bridgecircuit and thereafter transforming the rectified output thereof to highcurrent, low voltage power using a transformer means having a very lowratio of secondary to primary windings. Furthermore, it should beobvious that an inverting bridge stage could be connected to the outputof the full wave rectifying bridge circuit disclosed in FIGS. 3A and 38.Such an additional bridge could have switching elements of itsrespective arms located in the same paramagnetic salt control means usedby the rectifying bridge so that the overall circuit still utilized onlytwo superconductive switches which in this case would contain foursuperconducting switch elements each.

Further modifications of the present circuitry will occur to those ofordinary skill in the art upon perusal of the present cryogenicapparatus. Therefore, it should be understood that the aforementionedmodifications are intended only as examples and should in no way beconstrued to limit the instant invention. Thus, it is seen that a powersupply has been provided for cryogenic devices wherein joule heatinglomes have been substantially reduced due to the utilization ofadiabatic switching devices therein.

The embodiment of the instant invention which is depicted in FIG. 4A isa superconductive chopper or DC to AC converter, which admits of veryhigh frequency operation. As the illustrated apparatus is continuouslyrapidly cycled between its normal and superconductive states, it hasbeen found to possess the additional attribute of being able to operatein a loosely controlled cryogenic environment. Such operation is oftenadvantageous as the temperature of the cryogenic environment under theseconditions need not be constantly monitored or maintained within a fewtenths of a degree, as is the case when it is desired to magneticallyswitch a superconductive element across a narrowly defined, preselectedtemperature range.

The superconductive chopper or DC to AC converter depicted in FIG. 4Acomprises a controlled element 80, a control element 82, andparamagnetic salt means 84; all of which are maintained in anappropriate cryogenic environment which here has not been shown. Thecontrolled element 80 may take an appropriate shape or form andpreferably includes first portions 86 made of a relatively hardsuperconductive material having a critical temperature substantiallyabove that of the environmental temperature and a second portion 88which is made of relatively soft superconductive material having acritical temperature slightly above that of the environmentaltemperature. Thus, the first portions 86 of the controlled element 80are continuously in their superconductive state while the state of thesecond portion 88 thereof is determined, as shall hereinafter be seen,by the control ele meat 82 and the condition of the paramagnetic saltmeans 84. A source of DC potential 90 is connected to a first, inputterminal 92 of the controlled element 80 via the switch 11 and thesecond, output terminal 94 thereof is connected to a cryogenic load orutilization device which is generally indicated.

The control element 82 may be an ordinary current carrying conductor,however, since it is desirable to avoid joule effect heating within thesystem, a hard superconductive material similar to that used with regardto first portions 86 is preferred therefor. The control element 82 isconnected at a first terminal 96 thereof to a source of alternatingcurrent 98 which is here indicated as a square wave generator, but whichmay take any convenient form. The second or output terminal 100 of thecontrol element 82 is connected at G to ground as shown.

The paramagnetic salt means 84, which may be a single crystal grown onthe controlled element 80, is interposed between the control element 82and the controlled element in the vicinity of the second, softsuperconductive portion 88 thereof and is in intimate thermalcommunication with said second portion 88. The controlled element 80 ispositioned in a nonparallel, partially overlapping relationship with thecontrol element 82, which relationship is preferably perpendicular sothat the magnetic field produced by said control element 84 optimumlylinks said controlled element 80 as well as the paramagnetic salt means84. The overlapping portion of the controlled element 80 and the controlelement 82 having the paramagnetic salt means 84 interposed therebetweenis preferably encapsulated in a thermally insulative material 102 whichmay, for example, be Tefion. As will be described in more detailhereinafter, the insulating coating provides imperfect insulation whichallows the encapsulated elements to slowly approach the temperature ofthe external environment but provides sufiicient insulation so thatshort duration temperature changes are isolated therefrom, therebyallowing substantially adiabatic operation.

In operation the superconductive chopper or DC to AC converterillustrated in FIG. 4 is initially allowed to equalize in temperaturewith the cryogenic environment when the alternating current source 98and source of DC potential are in the deenergized condition. As theTeflon encapsulated portion thereof is substantially insulated from itsenvironment by the Teflon coating 102 as well as the segments of thecontrolled element 80 and the control element 82 external to said Tefloncoating, which segments are in the superconductive state and hence poorheat conductors, an appropriate time interval should be provided so thatthe encapsulated apparatus can approach the temperature of the cryogenicenvironment. Thereafter, the depicted apparatus is ready for operationas a superconductive chopper as both the first 86 and second 88 portionsof the controlled element 80 will be in the superconductive state andthe control element 82, in the preferred embodiment, will be in thesuperconductive state so long as the alternating current source 98remains deenergized.

The mode of switching of the apparatus illustrated in FIG. 4A may bebest explained in conjunction with FIG. 4B which is a plot of thresholdfield versus temperature for a superconductor material which may beassumed to be of the type utilized in second portion 88 of thecontrolled element 80. As can be seen by inspection of the figure, thearea under the curve denotes the superconductive regions of the materialwherein the material will exhibit essentially zero resistance andadditionally will be diamagnetic to fields of lesser value than thecritical field strength. The area above the curve represents the normalresistance state of the material where it is not diamagnetic. If such amaterial is switched in the usual manner from its superconductive stateto its normal state by the application of a large magnetic field H,which comfortably exceeds the critical field strength thereof H thechange in state is accomplished at a constant temperature T, and hencethe transition will follow the depicted path ABC. This manner ofswitching, which is commonly in use today, relies on the Meissner effectwhereby superconductivity is destroyed by the application of a magneticfield which has a sufi'icient magnitude to overwhelm the Meissnerbarrier of the superconductive material. However, such commonly usedswitching techniques require the application of a substantial field Hand even when this substantial field is utilized, the cryogenicenvironment must be maintained within a narrowly defined range asindicated by A T,.

If, however, paramagnetic salt means are placed in intimate thermalcommunication with the superconductive element which is to be switched,and further if such paramagnetic salt means are positioned so as to havethe domains therein aligned by the magnetic field applied to switch thestate of the superconductive element, a smaller magnetic field may beutilized as the transition in conductive state does not occur at aconstant temperature. Thus, as previously described, when theparamagnetic salt means are placed under the influence of a magneticfield, the randomly oriented magnetic domains therein will tend to alignthereby adiabatically releasing energy to the insulated volume in whichthe controlled element resides and increases the temperature thereof.The critical temperature and field is thereby shifted on the curve shownin FIG. 43 to T and H respectively whereby the path ADE is followed inswitching from the superconductive to the normal state of conductivity.Therefore, it will be seen that the introduction of the paramagneticsalt means introduces a thermal spike to aid in switching across theMeissner barrier thereby enabling a smaller field H to be applied inswitching the controlled element 80 of the superconductive apparatusdepicted in FIG. 4A. Further, the limits to which the temperature of theenvironment must be controlled have been substantially increased to avalue readily obtainable by the state of the art as indicated by A T dueto the utilization of the thermal spike produced by the paramagneticsalt means to augment the magnetic switching. It should be noticed thatthe limits of temperature control ofthe external environment may befurther increased by an increase in the applied field H and the converseof this situation also holds true above the curve.

The augmented switching principles described above have been relied uponin the chopper or DC to AC converter apparatus depicted in FIG. 4A.Thus, after the encapsulated portions of the apparatus illustrated inFIG. 4A have been allowed to equalize to the temperature T, of thecryogenic environment, the source of DC potential 90 and the source ofalternating current 98 are energized. Upon the energization of thesource of alternating current 98 a signal having the waveform of asquare wave is applied to the control element 82. The magnitude of eachcurrent pulse applied by the source of alternating current 98 issufficient to generate a field of a value H which exceeds the criticalfield strength necessary for the thermally augmented switching asdescribed above. Thus, with each current pulse applied by the source ofalternating current 98, the second, relatively soft portion 88 of thecontrolled element 80 is driven into the nonsuperconductive area abovethe FIG. 4B curve by the thermally augmented switching action of thefield H generated thereby. As the field H tends to align the randomlyorientated domains within the paramagnetic salt means 84, therebyreleasing the energy which creates the thermal spike, and the volumewithin which this energy is released is insulated from the cryogenicenvironment by the coating 102, the superconductive member 82 and thesuperconductive portions 86; the raise in temperature within theenclosed volume will be maintained for a period of time. Thus, duringthis period of time, the temperature of the volume will aid inmaintaining the second portion 88 in the normal state so that the entiremaintaining force need not be provided by the field H, which cantherefore be, if desired, below the necessary critical field strength.Further, as the released energy was retained within the insulated volumewhen the second portion 88 was driven normal, such energy will beavailable in the requisite amount to provide for the increased energystate of said paramagnetic salt means 84 when the domains therein tendto regain their random orientation due to the release of the magneticfield at the termination of a given current pulse. Thus, a negativethermal spike will be available to return the second portion 88 thereofto the superconductive range. Therefore, if the state ofthe second, softsuperconductive portion 88 is cycled within this time period, theswitching of the paramagnetic salt will be substantially adiabatic sothat no net increase in thermal energy is present.

As the frequency of the current pulses supplied by alternating currentsource 98 can be made very large with respect to the aforementioned timeperiod, due to the instantaneous switching which enables high frequencyoperation, the second soft superconductive portion 88 of the controlledmember 80 is adiabatically and instantaneously switched into the normalstate by the leading edge of each pulse supplied by the alternatingcurrent source 98 and instantaneously and adiabatically switched backinto the superconductive state by the trailingedge of each of saidpulses,

ince the second portion 88 of the controlled member is therebyalternately switched between its normal state and its superconductivestate by the field applied by the control element 82 and the thermalspike provided by paramagnetic salt means 84, the potential applied toinput terminal 92 of the controlled element by the source of potentialwill alternately be applied to the cryogenic load connected to outputterminal 94 thereof. Furthermore, this alternating potential will havesubstantially the same frequency as the alternating current source 98because the switching of the second, soft superconductive portion 88 issubstantially instantaneous. In addition, as the potential source 90 maybe a high voltage, low current source, no field which causes substantialinterference with the operation of the depicted DC to AC converter willbe produced; however, if a high current application is required, thealternating voltage produced thereby may be later transformed in themanner suggested with regard to the preceding embodiment.

Thus, it will be seen that a high frequency chopper has been provided inaccordance with the teachings of the instant invention.

While the invention has been described in connection with severalpreferred embodiments thereof, it will be understood that manymodifications will be readily apparent to one of ordinary skill in theart; and that this application is intended to cover any adaptations orvariations thereof. Therefore, it is manifestly intended that thisinvention be only limited by the claims and the equivalents thereof.

What is claimed is: l. A cryogenic switch comprising: means including anelement capable of exhibiting superconductivity and having a first stateat a first temperature and a second state at a second temperature;

paramagnetic salt means in intimate thermal communication with saidelement means and controlling the state thereof;

means to apply a magnetic field to said paramagnetic salt means, saidparamagnetic salt means, upon the energization of said magnetic fieldapplying means, switching said element means from one of said states tothe other of said states due to the magnetocaloric characteristicsexhibited thereby, said means to apply a magnetic field to saidparamagnetic salt means including a member capable of carrying currentspatially overlapping a portion of said means including an elementcapable of exhibiting superconductivity, said paramagnetic salt meansbeing interposed between said member capable of carrying current andsaid means including an element capable of exhibiting superconductivityat the overlapping portions thereof; and

means to thermally insulate said overlapping portions and saidparamagnetic salt means from thermal conditions external thereto.

2. The apparatus of claim 1 wherein said means to thermally insulatecomprises insulating material coated on said overlapping portions.

3. The apparatus of claim 2 additionally comprising means adapted tosupply voltage to said means including an element capable of exhibitingsuperconductivity and an alternating current to said member capable ofcarrying current whereby said current carrying member controls the stateof said element capable of exhibiting superconductivity.

4. The apparatus of claim 3 wherein said member capable of carryingcurrent comprises a rodlike superconductive member positionedperpendicular to a major axis of said means including an element capableof exhibiting superconductivity.

1. A cryogenic switch comprising: means including an element capable ofexhibiting superconductivity and having a first state at a firsttemperature and a second state at a second temperature; paramagneticsalt means in intimate thermal communication with said element means andcontrolling the state thereof; means to apply a magnetic field to saidparamagnetic salt means, said paramagnetic salt means, upon theenergization of said magnetic field applying means, switching saidelement means from one of said states to the other of said states due tothe magnetocaloric characteristics exhibited thereby, said means toapply a magnetic field to said paramagnetic salt means including amember capable of carrying current spatially overlapping a portion ofsaid means including an element capable of exhibiting superconductivity,said paramagnetic salt means being interposed between said membercapable of carrying current and said means including an element capableof exhibiting superconductivity at the overlapping portions thereof; andmeans to thermally insulate said overlapping portions and saidparamagnetic salt means from thermal conditions external thereto.
 2. Theapparatus of claim 1 wherein said means to thermally insulate comprisesinsulating material coated on said overlapping portions.
 3. Theapparatus of claim 2 additionally comprising means adapted to supplyvoltage to said means including an element capable of exhibitingsuperconductivity and an alternating current to said member capable ofcarrying current whereby said current carrying member controls the stateof said element capable of exhibiting superconductivity.
 4. Theapparatus of claim 3 wherein said member capable of carrying currentcomprises a rodlike superconductive member positioned perpendicular to amajor axis of said means including an element capable of exhibitingsuperconductivity.